Automatic frequency control system



July 20, 1948- M. 1 DoELz 2,445,663

AUTOMATIC FREQUENCY CONTROL SYSTEM Y Filed Jan. 19, 1946 /VEL v//V l. DQELZ IN VEN TOR.

Patented July 20, 1948 AUTOMATIC FREQUENCY CONTRL SYSTEM Melvin L. Doelz, Minneapolis, Minn..,assignor .to Collins Radio Company, Cedar Rapids, Iowa, a

.corporation of "Iowa Application January 19, 1-946, :Serial No. 642,214

Claims. 1

.This invention relates to automatic tuning control :systems and more particularly to a ,method 1.and .means for .controlling the frequency of .an :adjustable .frequency Aelectronic oscillator.

In certain types of yhigh frequency signalling systems employing an adjustable 4frequency `oscililator, 4itis requisite that the .oscillator be lstabilized `ata-ny -one of its predetermined frequency settings, .and oscillator dr-ift must .be reduced .to a minimum. Drift may result in ^an oscillator operating `at a `frequency either slightly above or slightly below the .desired frequency setting. ,In order to restore the oscillator automatically to .the desired frequency, it is ,necessary to employsorne tuning .control means which `will .respond not .only lto the magnitude .of drift but also 1to direction or Ipolarity of drift.

Accordingly, it is .a fprincipal object of this invention to provide 1a frequency responsive system which will immediately correct any .undesired .frequency deviations of an adjustable frequency oscillator.

It is .another object of .this invention to provide electromechanical ,control means for overvcoming oscillator frequency drift Vin .either direction with respect to the desired frequency setting of the oscillator.

It is another object of this invention to provide an electronic control circuit for controlling the direction and .dur-ation of rotation of a drift con- .trol motor.

It is a further object of this invention to provide an automatic oscillator tuning system wheren the direction of rotation of a tuning motor is determined -by a plurality .of electronically voperated relays and wherein an electronic circuit determines the direction in which said tun- -ing motor must be rotated to retune the oscillator, said direction of rotation being determined by the frequency .of the energy which the fre-v .quency .drift introduces into the system.

One of the features of this invention is .a unique circuit arrangement which is non-operative when the oscillator is on frequency and which actuates instantly a rotatable oscillator tuning control motor when the oscillator begins .to drift .and if the initial direction of rotation of the Ino- .energy introduced into the electronic circuit 'from the frequency drift of the oscillator is fed into a differentiating circuit which determines whether .2 the frequency ,is .increasing or decreasing while .the Vmotor is rotating .the tuning `means of .the .adjustable frequency oscillator.

.It .is .a l.further .feature of this invention that .a .positive or increasing .output voltage Lresults from .the differentiating .circuit if the .tuning motor .tends to increase the frequency .deviation lof the oscillator, and ,this increasing output voltage venergizes .a relay which causes .the ,tuning `motor 4to `reverse its .direction .of rotation so as .to

retune .the oscillator .to the desired .frequency setting.

Other ,features and advantages .not particularly enumerated will become apparent afterpa `consideration vof .the following detailed descrip tion and the Vappended claims.

The .drawing is `4a schematic wiring diagram .which shows .one preferred .manner of practising the invention.

Referring .more particularly to vthe drawing., the

4adjustable frequency .oscillator AI Il includes a I:tun-

.able oscillatory .circuit H anda @grid-.controlled .vacuumvtube-such as triode l2. The A.frequency -of voscillation of the circuit lll .is determinedhy yvary-ing the induotance L3 .forlarge increments known radiotransmitter 22, the other ,portionis fed fromplate 20 to grid ll. The electronic calibrator Y24 .provides a second sourceof `radio frequency energy of 'knownaccurate .frequency such .asa piezo .crystal-controlled oscillator. yThe output .of .calibrator 2'4 is fed to the second control .grid 25 .of mixer tube 1B. Thecalibrator '24 may be a multi-vibrator or other well-known means 'by which stable radio frequency waves `suitable for calibration .are produced.

The audio-.frequency transformer '26 applies the beat frequency output .from plate `21 to con- .tro'l grid 28 of audio-frequency amplifier tube 29. When the frequency ofthe calibrating waves impressed on .grid 25 is .the same `as the ouputfrequency of the oscillator I0 impressed on gridrl there will be no audio frequency vbeat note ,present in the output of mixer I9.

If the frequency of the adjustable frequency oscillator .I0 changes for anyreason, .an audio "beat 'frequency appears `across the secondary '30 `frequency transformer 5 I.

of transformer 26. In accordance with the invention, the plate current of tube 29 is made proportional to the frequency deviation of oscillator I0. For this purpose, resistor 3l and inductance l32 form a frequency responsive network which acts upon any audio-frequency signals applied thereto, so that when the output of tube 29 is substantially zero there is no Ibeat frequency and the output increases as the beat frequency increases. The audio frequency output of tube 29 may be further amplified by the amplier tube 33.

Control grid 34 of amplifier tube 35 is normally biased negatively to prevent plate current conduction through that tube. However, when an audio beat frequency appears across the secondary of transformer 26, `the negative bias on grid 34 is overcome sufficiently to cause platel current to flow through tube 35. rIhe winding of electromagnetic relay 31 is included in the plate circuit of tube 35, and this relay thereupon operates. The closing of the contacts of relay 31 completes a circuit traceable from the positive D. C. terrninal 38, conductor 39, winding of relay 49, conductor 4I, thence to the negative terminal v42. Relay 49 thereupon closes the power circuit from the line terminals 43, 44, to the armature of the motor. At the same time, the field winding 45 of the motor is connected in parallel with the armature 46, through the armatures 41, 48,

and the respective back contacts of directional control relay 49. The motor rotates the adjustable element I4, and lthereby changes the frequency of the oscillator I and the signals fed to grid I1. If the direction :of rotation of the motor is not such as to restore the oscillator I0 to its proper frequency as determined by the calibrated setting of device 24, additional means are provided to insure that even though the `motor should begin to rotate in the wrong direction, its direction of rotation is substantially im- Vmediately reversed to cau-se it to rotate to bring Ithe oscillator back to the proper frequency. For

this purpose, the output of amplifier 33 is also applied to the rectifier tube U through audio the motor is rotating in the wrong direction, an increasing D. C. current flows through the primaryof transformer 52; while if the direction of motor rotation is conrect, a decreasing current fiows through this primary. This change of curyrent through transformer 52 causes either a. l.positive o-r a nega-tive voltage to appear at grid 53 of amplifier tube 54. Tube 54 has its control grid vnormally biased so as to prevent the operation of relay 55 when a zero or negative voltage appears at transformer 52. On the other hand, when a positive voltage appears at that transformer,.suiiicient plate cunrent iiows through tube 54 to operate relay 55. The instant relay 55 operates, it immediately closes a locking circuit for itself traceable from positive terminal 38, winding of relay 55, armature 59, contact 60,. to the negative terminal 42 through the contacts of It will be noted therefore, that this is operated, that is to say, so long as any beat frequencies appear at transformer 26. Likewise, this locking circuit remains independent of the rectifier 55 and the` amplifier tube 54.

When relay` 55 operate-s, it closes a circuit traceable from the negativeV terminal 42, contact 51 and associated armature 56, winding of relay reverses the connections from the power line In other words, if .A

terminals 43, 44, to the eld winding 45, thereby reversing the direction of rotation of motor 34, and causing it to tune the oscillator towards its proper frequency. As the motor continues to rotate in the proper direction, the frequency deviation of oscillator III with respect to calibrator 24, is correspondingly reduced. This results in a decrease in the beat frequency appearing at transformer 26. When the frequency deviation approaches zero, insufficient current ows through relay 31 with the result that this relay opens :its contacts and therefore opens the locking circuit of relay 55. Relay thereupon releases as does relay 49, thus opening the power circuit lto the motor. From the foregoing, it will be seen that the relay 55 is in effect a directional sensitive relay. That is to say, it is initially energized only when the motor is actually rotating in the Wrong direction tending to increase the frequency deviation o-f the oscillator.' I-Iowever, once energized, it remains locked until the deviation is reduced to zero. It provides therefore a positive check or limit on the amount of undesirable initial rotation of the motor and at the same time it provides a positive automatic control of the reversal of direction of the motor so as to reduce to zero the frequency deviation of the oscillator with respect to calibrator 24.

It is desirable to have some visual frequency indicating means which may be used to facilitate the manual tuning of the oscillator and mixer stages. Therefore, any suitable frequency indicating device such as a calibrated dial may be suitably mechanically connected to the shafts of the adjustable elements I3 and I5. After accurate tuning is lonce accomplished by adjusting manually elements I3 and I5, the automatic control means above described will keep the oscillator I0 in accurate adjustment.

While there has been here described a preferred embodiment of this invention, it is understood that various changes and modifications may be made therein without departing from the scope of the invention, as defined in the following claims.

What is claimed is:

1. In combination, an oscillator for generating a voltage of adjustable frequency, a source for producing another voltage of a certain standard frequency, a mixer network upon which signals from said oscillator and said source are simultaneously impressed to produce an oscillator tuning control voltage proportionate to the frequency difference between said oscillator and said source, a motor for controlling said oscillator, a directional control device for determining the direction of rotation of said motor, a relay arrangement responsive to said tuning control voltage above a predetermined minimum to cause said motor to rotate, and another relay arrangement responsive to an increase in said tuning control voltage resulting from the starting of said motor in the wrong direction to operate said directional control device to reverse the motor rotation until said oscillator is at the desired frequency.

2. In combination, an oscillator for generating a voltage of adjustable frequency, a source for producing another voltage of a certain standard frequency, electron tube mixer means for combining said voltages to produce a tuning control voltage which is proportionate to the difference between the frequency of the voltages from said oscillator and from said source, a first relay which responds to .Said timing control voltage above 'a predetermined minimum, a secondrelay adjustment and the extent of the adjustmentof said adjusting means.

3,1m combination, an oscillator for generating va voltage of ladjustable frequency, a source'for producing: another voltage of va certain standard frequency, means lto mix said'voltages to Yproduce atuning control voltage which is'proportlonate to the'difierence between the frequency of said oscillator and that of said source, the last-mentioned means including a'frequencyl dis- :crixninator network, a relay connected to said network which responds .to said voltages above a predetermined minimum, means to rectify said voltages `to produce a uni-directional voltage onlywhen said first-mentioned voltage is varying, another relay responsive to said uni-directional voltage andfwhose energizing lcircuit is controlled by said uni-directional voltage, a locking vcircuit for said second-mentioned relay which locking circuit is independent of said uni- Adirectional'voltage vbutiscontrolled by the iirst- ,mentioned relay, means to adjust a movable tuning element of `said oscillator, and a motor device for controlling the direction and extent of said adjustingmovement, said motor device being controlled jointly by said first and secondmentioned relays.

4. In afrequency control system, a frequency combining network to derive a `tuning control voltage Iproportionate to the deviation of an oscillator from a standard frequency, circuit arrangements including a relay which responds to said voltage above a predetermined minimum, a motor for adjusting said oscillator, a starting control relay and a direction control relay for controlling the power circuit of said motor, circuit connections for applying said voltage to `causeone of said pair of relays Vto control the initial starting of said motor, and for'causing the other of said pair of relays to control the direction of rotation of said motor, means to operate the said starting control relay in response to the operation of the first-mentioned relay, and means to operate the direction control relay only when said voltage tends to increase as a result of the starting of the operation of said motor in the wrong direction.

5. A frequency control system according to claim 4 in which an additional relay is provided for controlling the initial energization of said direction control relay, said additional relay being energized only when said voltage tends to increase as a result of the starting of rotation of said motor in the wrong direction.

6. In a frequency control system, a frequency combining network to derive beat frequencies corresponding to the output of an adjustable oscillator and a standard frequency source, a frequency discriminator network including an electron discharge tube amplifier upon which said beat frequencies are impressed, a relay which is energized by the output of said amplifier above a predetermined minimum, a rectier for rectifying a portion of said beat frequencies, a gridcontrolled amplifier tube normally biased substantially to plate current cutoff and responsive to positive rectied voltages from said rectifier to "producefplat'e current flow, a second-relay which-is initially energized in response to said plate currentffiow, a'locking circuit for said secondrelay including contacts of the iirstfrelay which locking `circuit is independent of said rectified beat'frequencies, a motor control relay which isoperated'under control of said first relay, a motor reversing relay whichis operated in response to the operation of said second` relay, and circuit arrangements for causing motor control relay to be operated only when the initial direction. of' rotation of said motor is such as Yto lincrease said beat frequencies, so as to reverse'the rotation of said motor to a direction tofreduce said beat frequencies. Y

`v'7. In la frequency control system, an electron tube oscillator having :a frequency-determining circuit connected thereto, a source of standard frequency oscillations, a mixer tube having -a tunable input circuit connected to one control grid thereof, said input circuit being'tuned in unison with the tuning of said frequency-determining circuit, means to impress said standard frequency oscillations on another control grid of said mixer tube to produce audio-frequency beats in the Voutput of said mixer tube corresponding to the deviation between the frequency of said oscillator and said standard source, frequency discriminatcr network connected to the output of said mixer' tube,va grid-controlled amplifier tube normally biased to plate current cutoff, and means connecting the control grid of said tube to the output of said net-work to produce plate current when the output of -said network is above a predetermined minimum, an electromagnetic relay controlled by the' plate'current of said grid-controlled. tube, a motor starting relay controlled by said electromagnetic relay. a second grid-controlled tube normally biased substantially to plate current cutoff, means to produce positive D.: C. voltages proportionate to the output of said amplifier, means toV apply vsaid positive'D. C. voltages to the grid of said second grid-controlled tube `to cause plate current to flow therethrough, another electromagnetic relay which is initially energized in response toan impulse of plate `current through said second grid-controlled tube, a locking circuit for said second electromagnetic relay which is closed through the contacts of the first-mentioned electromagnetic relay so as to be independent of said second-mentioned grid-controlled tube, and a motor reversing relay which is controlled by said second-mentioned electro-magnetic relay.

8. The method of adjusting the frequency of an adjustable oscillator which comprises, producing a voltage which is proportionate to the deviation of said oscillator from a known standard frequency, operating a tuning element of said oscillator in response to said Voltage, producing another voltage which corresponds to an increase in said deviation, and applying the second-mentioned voltage to determine the direction of operation of sai-d tuning element,

9. In a frequency control system, means to produce beat frequencies corresponding to the frequency deviation of an adjustable oscillator with respect to a standard frequency source, a iirst electromagnetic relay responsive to the presence of said beat frequencies, a motor for adjusting a tuning element of said oscillator, a second electromagnetic relay for controlling the starting of said motor, a third electromagnetic relay for controlling the direction of rotation of said motor, and a fourth electromagnetic relay for determining the duration of rotation of said motor,l said relays being interconnected so that said motor responds initially to the presence of said beat frequencies and its direction of rotation is automatically corrected so as to reduce said beat frequencies substantially to zero.

10. An oscillator system including an adjustn able frequency electron tube oscillator, including an adjustable frequency-determining element, a motor connected to said element to vary the oscillator frequency, a source of standard frequency, a frequency mixer adapted to` produce a Zero beat frequency only when the signal received by it from said oscillator is substantially equal to the signal received by it from said standard source, a frequency sensitive network having an output which varies in correspondence with the frequency of the energy received by it from said mixer, a control circuit connected to the output of said frequency sensitive circuit', said control -circuit being effective when said mixer produces an output to complete the power circuit to said motor, said control circuit also including a motor reversing control relay, `directional differentiating means comprising an electron tube arrangement 'for converting a part of the output of said network into a voltage whose polarity is dependent upon whether said motor is rotating in a direction tending to produce said zero beat frequency, or it is rotating in a direction tending to increase said beat frequency, and means to operate said reversing switch' contacts only when said motor starts to rotate in a direction tending to increase said beat frequency.

11. A system according to claim 10 in which said directional differentiating means includes a rectier for rec'tifyiner a portion of the output of said frequency-sensitive network, and a gridcontrolled electron tube which is normally blassecl to plate current cuto and which remains at plate current cutoff only while the output'of said recti er is decreasing.

12. A system according to claim 10 in which said reversing control relay is connected in the output of said electron tube arrangement.

13. An automatic frequency-controlled electronic oscillator system including an oscillator for generating a voltage of an adjustable frequency, a source for producing another voltage of a cer'- tain standard frequency, a tuning motor for said oscillator,v a frequency mixer connected .to said oscillator and source to produce a beat frequency corresponding to the deviation of said oscillator 'from a pre-assigned frequency, a beat frequency responsive circuit arrangement connected to the output of said mixer and effective to cause operation of said motor only when a beat frequency exists at the output of said mixer, said beat frequency responsive circuit including a motor starting relay which closes the motor circuit in response to a beat frequency and a motor directional control relay which responds only when' the motor is started in a direction tending to" increase said beat frequency.

14. A system according to claim 13 inwhich said motor start relay has a locking circuit which is completed through the normally open contacts of said directional contro1 relay.

15. In an automatically tuned oscillator system, means to produce a variable beat frequency corresponding to the departure of the oscillator from a pre-assigned frequency, an electron tube net'- w'ork to which'said beat frequency is appliedfor producing in its output a control signal whose amplitude varies with' the frequency ofthe beat, a motor for tuning the oscillator, a relay which operates in response to the presence of a beat frequency, another relay connected to the -output of said network and operated only when the motor rotates in a direction tending to increase said beat frequency, and a third relay for controlling the direction of rotation of the motor, said third `relay being operated under the joint control 'of the first and second relays.

. MELVIN L. DOELZ.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,873,842 Hyland Aug. 23, 1932 2,005,153 Maries Jan. 18, 1935 

